Do not warn by default when policy CMP0025 or CMP0047 is not set

These policies are triggered by the use of a particular compiler rather
than outdated CMake code in a project.  Avoid warning in every project
that enables a language by not displaying the policy warning by default.
Add variable CMAKE_POLICY_WARNING_CMP<NNNN> to control the warning
explicitly; otherwise enable the warning with --debug-output or --trace.

This breaks with strict policy convention because it does not provide
developers with any warning about the behavior change by default.
Existing projects will continue to build without a warning or change in
behavior.  When a developer changes the minimum required version of
CMake in a project to a sufficiently high value (3.0), the project will
suddenly get the new compiler id and may break, but at least the
breakage comes with a change to the project rather than the version of
CMake used to build it.

Breaking strict policy convention is worthwhile in this case because
very few projects will be affected by the behavior change but every
project would have to see the warning if it were enabled by default.

Help/manual/cmake-variables.7.rst

@@ -131,6 +131,7 @@ Variables that Change Behavior
    /variable/CMAKE_MODULE_PATH
    /variable/CMAKE_NOT_USING_CONFIG_FLAGS
    /variable/CMAKE_POLICY_DEFAULT_CMPNNNN
+   /variable/CMAKE_POLICY_WARNING_CMPNNNN
    /variable/CMAKE_PREFIX_PATH
    /variable/CMAKE_PROGRAM_PATH
    /variable/CMAKE_PROJECT_PROJECT-NAME_INCLUDE

Help/policy/CMP0025.rst

@@ -18,6 +18,10 @@ to the invocation of either command.
 The OLD behavior for this policy is to use compiler id ``Clang``.  The
 NEW behavior for this policy is to use compiler id ``AppleClang``.
 
-This policy was introduced in CMake version 3.0.  CMake version
+This policy was introduced in CMake version 3.0.  Use the
-|release| warns when the policy is not set and uses OLD behavior.  Use
+:command:`cmake_policy` command to set this policy to OLD or NEW explicitly.
-the :command:`cmake_policy` command to set it to OLD or NEW explicitly.
+Unlike most policies, CMake version |release| does *not* warn
+by default when this policy is not set and simply uses OLD behavior.
+See documentation of the
+:variable:`CMAKE_POLICY_WARNING_CMP0025 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
+variable to control the warning.

Help/policy/CMP0047.rst

@@ -19,7 +19,10 @@ The OLD behavior for this policy is to use the ``GNU`` compiler id
 for the qcc and QCC compiler drivers. The NEW behavior for this policy
 is to use the ``QCC`` compiler id for those drivers.
 
-This policy was introduced in CMake version 3.0.
+This policy was introduced in CMake version 3.0.  Use the
-CMake version |release| warns when the policy is not set and uses
+:command:`cmake_policy` command to set this policy to OLD or NEW explicitly.
-OLD behavior.  Use the :command:`cmake_policy` command to set it to
+Unlike most policies, CMake version |release| does *not* warn
-OLD or NEW explicitly.
+by default when this policy is not set and simply uses OLD behavior.
+See documentation of the
+:variable:`CMAKE_POLICY_WARNING_CMP0047 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
+variable to control the warning.

Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst

@@ -0,0 +1,17 @@
+CMAKE_POLICY_WARNING_CMP<NNNN>
+------------------------------
+
+Explicitly enable or disable the warning when CMake Policy ``CMP<NNNN>``
+is not set.  This is meaningful only for the few policies that do not
+warn by default:
+
+* ``CMAKE_POLICY_WARNING_CMP0025`` controls the warning for
+  policy :policy:`CMP0025`.
+* ``CMAKE_POLICY_WARNING_CMP0047`` controls the warning for
+  policy :policy:`CMP0047`.
+
+This variable should not be set by a project in CMake code.  Project
+developers running CMake may set this variable in their cache to
+enable the warning (e.g. ``-DCMAKE_POLICY_WARNING_CMP<NNNN>=ON``).
+Alternatively, running :manual:`cmake(1)` with the ``--debug-output``
+or ``--trace`` option will also enable the warning.

Source/cmGlobalGenerator.cxx

@@ -759,7 +759,8 @@ void cmGlobalGenerator::CheckCompilerIdCompatibility(cmMakefile* mf,
     switch(mf->GetPolicyStatus(cmPolicies::CMP0025))
       {
       case cmPolicies::WARN:
-        if(!this->CMakeInstance->GetIsInTryCompile())
+        if(!this->CMakeInstance->GetIsInTryCompile() &&
+           mf->PolicyOptionalWarningEnabled("CMAKE_POLICY_WARNING_CMP0025"))
           {
           cmOStringStream w;
           w << policies->GetPolicyWarning(cmPolicies::CMP0025) << "\n"
@@ -790,7 +791,8 @@ void cmGlobalGenerator::CheckCompilerIdCompatibility(cmMakefile* mf,
     switch(mf->GetPolicyStatus(cmPolicies::CMP0047))
       {
       case cmPolicies::WARN:
-        if(!this->CMakeInstance->GetIsInTryCompile())
+        if(!this->CMakeInstance->GetIsInTryCompile() &&
+           mf->PolicyOptionalWarningEnabled("CMAKE_POLICY_WARNING_CMP0047"))
           {
           cmOStringStream w;
           w << policies->GetPolicyWarning(cmPolicies::CMP0047) << "\n"

Source/cmMakefile.cxx

@@ -4350,6 +4350,22 @@ cmMakefile::GetPolicyStatusInternal(cmPolicies::PolicyID id) const
   return this->GetPolicies()->GetPolicyStatus(id);
 }
 
+//----------------------------------------------------------------------------
+bool cmMakefile::PolicyOptionalWarningEnabled(std::string const& var)
+{
+  // Check for an explicit CMAKE_POLICY_WARNING_CMP<NNNN> setting.
+  if(!var.empty())
+    {
+    if(const char* val = this->GetDefinition(var.c_str()))
+      {
+      return cmSystemTools::IsOn(val);
+      }
+    }
+  // Enable optional policy warnings with --debug-output or --trace.
+  cmake* cm = this->GetCMakeInstance();
+  return cm->GetDebugOutput() || cm->GetTrace();
+}
+
 bool cmMakefile::SetPolicy(const char *id,
                            cmPolicies::PolicyStatus status)
 {

Source/cmMakefile.h

@@ -875,6 +875,8 @@ public:
   std::set<cmStdString> const & GetSystemIncludeDirectories() const
     { return this->SystemIncludeDirectories; }
 
+  bool PolicyOptionalWarningEnabled(std::string const& var);
+
 protected:
   // add link libraries and directories to the target
   void AddGlobalLinkInformation(const char* name, cmTarget& target);